Differentials are primarily aimed at adequately transferring rotational torque when there are differences on rotational speeds between the opposite output axle shafts of wheels on the automobile. Differential slip is a factor which causes the automobile to slip on the slippery surfaces such as the ice surface and rocks and eventually, the automobile stops moving. When the vehicle moves in a straight line, the right and left wheels spin together with the same speed, but when it turns, the wheels near the turn center traverse less distance and on the other hand, the wheels distant from the turn center makes a turn to cover more distance. Therefore, the wheels near the turn center should spin with less speed than the wheels that are away from the center of turn. To change the speed of the wheels of the vehicle especially in road turns, a complex called differential is used.
However, the open differential while retaining balance in wheels, is associated with more problems. The differential spins the wheel faster which has less inhibiting force. Therefore, if the friction of any wheel is reduced for any reasons, the wheel spins with more speed. This differential slip causes energy dissipation and instability when moving on the slippery surface and even stop of the vehicle. The problem is more in heavy vehicles and agricultural machineries, which move in uncommon routes. When dropped in the mud or being on the slippery surface of the ice, the friction of one of the wheels is reduced with the ground. The wheel with the less friction spins faster and consequently, the other wheel which has a proper friction with the ground spins slowly. This phenomenon could lead to the stoppage of the vehicle or it might get stuck. This problem could be easily noticed by jacking under one of the wheels and separating it off the ground. In this case, when the engine power is transferred to the wheels, the wheel is separated from the ground and receives the entire force and eventually the vehicle will not move.
Several systems and methods are available in the prior art to solve the limitations of the open differential in constraining the tire slip. One of the widely used technique is locking differentials. This type includes components like that of the open differentials, in addition, a pneumatic or hydraulic-electrical mechanism to lock the two output gears to each other. Commonly, this mechanism is activated by a switch, when activated, both wheels will spin with the same speed. Differential lock types comprise Non-spin locker differential, E-locker differential and gearless differential lock. In non-spin locker type, the engaging power is provided by a high-pressure air compressor. One of its prime advantage is problem free and quite convenient installation on various types of differentials. In E-locker type, the differential is locked by a strong magnet and the driver could activate differential lock by pressing a button from inside the car. However, this is an expensive model. Gearless differential lock is primarily used for light weight vehicles.
There are many mechanisms available in the prior art which uses limited differential slip. Limited differential slip is configured to limit the slip to an acceptable level. When one of the wheel slips, the differential allows more torque to be transferred to the wheel which does not slip. Some of the types are a) Clutched limited slip differential which is a commonly used type which includes all the differential components along with springs and clutches. Some of them have a conical clutch just as the coordinator in manual power transmission system. The spring pushes the lateral gears, which are connected to the casing, into the clutches. When the wheels move with the same speed, both lateral gears spin along with the casing and there is no need for clutches. Only when an agent causes one of the gears spin with more speed than the other, the clutch acts. If one of the gears wants to spin more quickly, it should first overcome the clutch. However, if one of the wheels is on the ice and the other one has sufficient friction to move, with limited slip differential, although the wheel on the ice is not able to transmit more torque to the ground. The other wheel still receives the torque required to move.
Another technique is viscous coupling wherein two sets of plates container in a chamber filled with thick liquid. Each set of plates is connected to one of the output shafts. The plate set, and the thick liquid rotate with the same speed but, when one of the wheels rotates faster, the plate set connected to this wheel and rotates more quickly than the other set of plates. The thick liquid trapped between the plates desires to rotate with the same speed as the faster and slower plates do. However, this differential is not effective, and limitation is that when one of the wheels starts to slip, no torque is transmitted.
Another technique is using torsion differential which is a completely mechanical device that does not include the use of electronics or clutched system or a thick liquid. When the torque transmitted to both wheels are same, it works as an open differential. The torque difference makes the gears to be constrained to each other in the Torsion differential. Major drawback in this type of differential is that when one of the wheels is separated completely off the ground, Torsion differential will not able to transmit any torque to other wheel. The reason for this issue is that tendency to torque change will determine the amount of the torque transmitted.
Electronic systems are also used for stability control by minimizing the automobile slip in turns. ASR and dynamic drive system using electronics are also used to retain stability by reducing the engine power or braking on wheels spinning redundantly. Sometime, the friction of the tires is improved to prevent the differential slip in turns. However, all the above-mentioned methods and systems have limitations in achieving the reduction of differential slip. Limitations such as complex and cumbersome design increases the production cost and lifetime is very less. Depreciation cost is also high and some of them are fully automatic which reduces the adaptability of the system. Some of the methods aren't successful in riding impassable areas. Most of the system uses clutch plates as a part of their design which further causes strain and wear on drive-train components along with causing greater energy consumption required to overcome the friction.
Most of the current techniques employing additional consumable mechanical components such as clutch plates, spring members, high pressure rotating seals, disc members and coupling device to adjust the amount of slip tailored for the conditions of the driven vehicle. However, these systems continue to cause more wear in the moving components which requires repeated maintenance and thereby increasing the cost. The depreciation level is also very low. Further, most of the designs are cumbersome and the reliability on the efficiency of the system is very minimal. Current techniques do not allow precise adjustment of the rate of differential for the slip in the vehicle based on the driving conditions.
Prior art reference U.S. Pat. No. 8,202,189 B2 discloses a limited slip differential wherein a driving plate, backing plates, differential gear assemblies, and a transmission assembly engaged together inside a sealed casing where fluid is pumped. The rotational speed difference of axle shafts of the vehicle exceeds a threshold value, the LSD applies at least one gear assembly to generate a back pressure and efficiently block the fluid passing through the gears for limiting mutual rotational speed difference, hence achieving a limited-slip effect. However, this design is complex and using more consumable parts such as driving and backing plates that causes more wear.
Thus, in light of aforementioned drawbacks, there is a clear and present need for a simple limited differential design in automobile vehicles to constraint the tire slip for increasing the stability of the automobile while driving on the slippery roads.